In recent years, a vehicle-to-vehicle-communications system is proposed. In a vehicle-to-vehicle-communications system, each of multiple vehicles sends a communication packet to another vehicle and receives successively a communication packet sent from another vehicle. The communication packet indicates vehicle information, such as a traveling speed, a present position, and a traveling direction of a primary vehicle of a driver. A driving assist device may be employed in such a vehicle-to-vehicle-communications system. The driving assist device may assist an operation of a driver according to vehicle information on a primary vehicle and vehicle information on another vehicle. The other vehicle may include multiple vehicles. The vehicle information on another vehicle is retrieved through vehicle-to-vehicle communications.
For example, Patent Document 1 discloses a technology to determine whether a primary vehicle makes physical contact with another vehicle according to both vehicle information on the primary vehicle and vehicle information on another vehicle. The vehicle information on the other vehicle is specified by a received communication packet. The technology of Patent Document 1 further warns a driver to avoid physical contact with the other vehicle on determination that the physical contact possibility occurs.
As described in Patent Document 1, the above-described vehicle-to-vehicle-communications system employs the carrier sense multiple access/collision avoidance (CSMA/CA) mechanism as an access control system.
The communication between a vehicle and a roadside equipment installed on a roadside may use same frequency as vehicle-to-vehicle communications, for example, using CSMA/CA.
(Patent Document 1)
Japanese Patent Gazette No. 5265027
A driving assist device of the primary vehicle receives a communication packet sent from a certain vehicle thereby to retrieve information about the vehicle. In addition, the driving assist device receives a communication packet from another vehicle, from which the driving assist device has not received a communication packet, thereby to recognize existence of the vehicle for the first time.
It is noted that, the vehicle-to-vehicle-communications system employs the CSMA/CA mechanism. Therefore, communication packets, which are sent from multiple vehicles, may collide against each other. Thus, there may be the case that the driving assist device of the primary vehicle cannot demodulate the received communication packet successfully.
In a state where the driving assist device has not demodulate a communication packet sent from a certain vehicle (first another vehicle) successfully yet, a communication packet, which is sent from the first another vehicle, may collide against a communication packet, which is sent from another vehicle (second another vehicle). In this case, the driving assist device cannot recognize existence of the first another vehicle continually for a time period.
Consequently, the driving assist device cannot notify a driver of existence of the first another vehicle, even though the first another vehicle exists at a position and may make physical contact with the primary vehicle.